In general It’s possible to create CityGML from Multipatch with Data Interoperability Extension. The thing is, that you have to make use of the CityGML writer instead of the GML writer. But by using only the Quick Export tool you will get some CityGML. I would strongly recommend to use an ETL tool (in your own ArcToolbox using Data Interoperability Extension) to create the export to CityGML. This is not straight forward, you have to set some CityGML specific attributes and properties to get ‘good’/ well-structured CityGML. If you like send me an email to c.dahmenATconterra.de and I'll send you a sample workspace (ETL tool).